Objective, particularly a projection objective for use in semiconductor lithography

ABSTRACT

The invention relates to an objective, particularly a projection objective for use in semiconductor lithography, comprising optical elements such as lenses ( 16 ) and mirrors ( 11   a ). According to the invention, at least a portion of the optical elements ( 16, 11   a ) is provided with a reflective surface outside of the optically active area serving as the reference surface ( 12 ) for adjusting the optical element inside the objective ( 8 ).

[0001] The invention relates to an objective, particularly a projection objective for use in semiconductor lithography, having optical elements such as lenses and mirrors.

[0002] When assembling an objective, particularly a projection objective in semiconductor lithography, for producing semiconductor elements, it is necessary for the individual objective parts such as, for example, lenses and mirrors, to be installed with very high absolute accuracy relative to one another as regards both the linear coordinates and the angular coordinates.

[0003] Optical elements used to date, for example mounts of lenses, have a centering collar on the outside diameter, and an upper and a lower surface with a reference and sealing collar as references for installation in the objective. A lens is aligned in this case such that its optical axis goes perpendicularly through the two reference surfaces of the mount. For this purpose, the lens is “rolled in” exactly with reference to its optical axis and then bonded to the mount. In this case, however, it is impossible to avoid an offset relative to the mechanical axis of the flange or of the mount. For this reason, the offset of the optical axis of the lens relative to the centering collar on the mount is measured for size and angular position, and then used to provide an appropriate lead during mounting into the objective. This is frequently sufficient, since each mount generally rests on the preceding one and then needs only to be centered laterally.

[0004] In the case of a catadioptric objective, however, it is necessary to measure the angular position of the individual lens groups relative to one another ever a relatively large distance between a central body and the mounts. Known measuring machines, for example portal or stand-mounted measuring machines having tactile measuring keys can be used for this purpose and are, if appropriate, further provided with an optical measuring system for absolute determination of location and angle. Plane mirrors are used in this case to measure the reference surfaces of the respective optical element.

[0005] It is provided to adjust a reference surface with the aid of a plane mirror in order to adjust lens groups of a catadioptric objective in a subassembly in which the optical axis runs at least approximately horizontally and with a vertical subassembly in which the optical axis runs in the vertical direction. The plane mirror must subsequently be exchanged for the respective lens group.

[0006] It is disadvantageous in this case that, owing to the exchange of the plane mirror or mirrors with the respective lens group, not only is a higher outlay on installation necessary, but, in addition, new sources of error can occur.

[0007] It is therefore the object of the present invention to avoid the abovenamed disadvantages for an objective, particularly to create an objective of the type mentioned at the beginning that simplifies the adjustment, the aim also being to reduce possible sources of error.

[0008] This object is achieved according to the invention by means of the features named in the characterizing part of claim 1.

[0009] It is possible by virtue of the reference surfaces according to the invention, which are arranged on the optical element itself or on the mount thereof, for a lens or mirror group to be directly adjusted, specifically without making a detour via plane mirrors and a subsequent exchange resulting therefrom, which can always lead to errors.

[0010] The reflecting surface, for example a mirror surface, integrated in or on the optical, elements can be produced simultaneously with the optical element or with the mount. According to the known gluing-in operation for a lens, given an appropriate arrangement, the reflecting surface can also lie parallel to reference collars of the mount and thus perpendicular to the optical axis.

[0011] The reflecting surface can therefore be used to undertake an exact adjustment of the respective objective part relative to one or more specific reference points in the objective. This can, for example, be performed by means of an optical measuring system with the aid of measuring beams produced by an interferometer or an autocollimation telescope. All that is required is to design the size of the reflecting surface in accordance with the desired accuracy of adjustment and the available accuracy of the measuring means.

[0012] It is also advantageous that the reference via the corresponding reference surfaces, for example mirror surfaces, is already present in the objective 8 for a later control of the angular position of the installed optical elements. A later control is thereby possible without a high outlay.

[0013] An exemplary embodiment of the invention is described below in principle with the aid of the drawing, in which:

[0014]FIG. 1 shows an illustration of the principle of a measuring machine for measuring and adjusting optical elements installed in a frame structure of an objective;

[0015]FIG. 2 shows a half-section through a lens having the reflecting surface according to the invention on the mount; and

[0016]FIG. 3 shows an illustration of the principle of a catadioptric objective.

[0017] A measuring machine 1, for example a portal measuring machine, of known design has a measuring table 2 as a granite block that has a vertical measuring bore 3 with a transverse bore 4 in the lower region. An autocollimation telescope 5 is flanged on at the end of the transverse bore 4. Of course, an interferometer can also be used instead of an autocollimation telescope 5. A deflecting mirror 6 is arranged at the point where the measuring bore 3 meets the transverse bore 4. The autocollimation telescope 5 can be calibrated at the start to the surface of the measuring table 2 as reference surface 7 with the aid of the deflecting mirror 6 and a plane mirror (not illustrated) that is laid on the surface of the measuring table 2 over the measuring bore 3. It is possible in this way for surfaces that are to be measured with the aid of the autocollimation telescope 5 always to be referenced in absolute terms relative to the measuring surface. The aim is to use the measuring machine 1 to measure the upper part of an objective 8, the optical elements being inserted into an upper frame structure 8 a and a lower frame structure 8 b.

[0018] The upper component of the objective is mounted with the upper frame structure 8 a on the measuring table with its reference surface 7 in a first step, in order to assemble the objective 8 and/or to install the optical parts of the objective. The underside of the frame structure 8 b likewise serves as reference surface 9.

[0019] A prismatic body 10 is inserted into the upper frame structure 8 a. An optical subassembly 11 having a curved mirror 11 a and one or more Senses 11 b is simultaneously flanged on at the side. Subsequently, the underside of the prismatic body 10 is aligned as auxiliary surface by means of the autocollimation telescope 5 (see the beam path a in FIG. 1 in this regard). The auxiliary surface is produced during optical fabrication with an appropriate average accuracy relative to the front surfaces of the prismatic body 10. The prismatic body 10 is aligned in this way with an appropriate accuracy within the horizontal plane.

[0020] Subsequently, the lens group 11 with the curved mirror 11 a is aligned relative to the autocollimation telescope 5 (see beam path b in FIG. 1). For this purpose, the optical beam emanating from the autocollimation telescope 5 must be retroreflected. This is done by widening the diameter of the curved mirror 11 a, which is located outside the optically active region, the point being that a reflected surface is formed on the widening of the diameter in the form of a mirror surface as reference surface 12. In other words: the reflecting surface is located in a region outside the region required for imaging. It is therefore necessary merely to select the beam path b such that it strikes the annular mirror surface 12. In this way, the entire optical subassembly 11 with its optically active surfaces can then be aligned with appropriate accuracy with reference to its angular position.

[0021] The required adjustment and/or the measurement of the distance of the optical subassembly 11 from the tip of the prismatic body 10 can be performed by a measuring head 13, which can be moved appropriately on the measuring table 3 in the direction of the arrow, of the measuring machine 1, and by known measurements with the aid of one or more tactile measuring elements 14.

[0022] A separate, exactly controlled lifting table 15 that grips the underside of the optical subassembly 11 and raises or lowers the latter as appropriate can be used for exactly displacing the optical subassembly 11 on the outer surface or an interface surface of the upper frame structure 8 a in accordance with the direction of action illustrated by an arrow.

[0023] Subsequently, a lens or lens group 16 is mounted on the upper frame structure 8 a. the lens 16 or, in the case of a lens group, the lens 16 situated next to the measuring table 2, is likewise provided, in the region of its mount 17, with a reference surface in the form of a mirror surface 12 as reflecting surface. The lens 16 is then aligned exactly by means of the mirror surface 12. All that is required for this purpose is to direct the measuring beam c appropriately onto the mirror surface 12.

[0024] As may be seen, it is possible in this way to measure and install the individual optical elements or the components of the objective 8 with high accuracy in an absolute fashion in terms both of location and of angle. This purpose is also served in a known way by sealing and reference collars 18 on the undersides and on the top sides of the mount 17 of lenses 16. The same also holds, of course, for the curved mirror 11 a of the subassembly 11. The reference collars 18, which have a vary high plainness surface, are situated exactly perpendicular to the optical axis 19. It is to be ensured, concerning the connection of the lens 16 to the mount 17, that bonding into and onto bearing surfaces 20 of the mount 17 is performed such that the optical axis 19 is situated perpendicular to the reference surfaces 18. Since the mirror surface 12 has been applied to one of the reference collars 18, for which purpose the corresponding reference collar 18 is widened to such an extent as to produce a sufficiently large annular surface for reflection of the measuring beams b and c, the mirror surface 12 is thus likewise situated perpendicular to the optical axis 19. In general, a width of approximately 5 mm will be sufficient for the mirror surface 12 (see enlarged illustration in FIG. 2). In order to displace the lens 16 laterally for the desired adjustment, it has, on the outer circumference, a centering collar 21 on which actuators (not illustrated) can act for displacement purposes.

[0025] The refractive part 22 installed in the lower frame structure 8 b can be measured and adjusted in a similar way. The assembly of the two frame structures 8 a and 8 b can be performed subsequently in any desired way.

[0026] Although the use of the mirror surfaces 12 for a curved mirror 11 a and the mount 17 of a lens 16 is described only for a measurement method with the aid of the measuring machine 1, it is, of course, clear that optical, elements provided with the reflecting surface according to the invention, for example the mirror surface 12, can also be used in another way within the scope of the invention for exactly adjusting and/or determining the location and angle of the respective optical element.

[0027] The mirror surface 12 can be taken into account appropriately, for example, in the fabrication as early as when producing the mount 17 and the lens 16 or the curved mirror 11 a. Thus, the mirror surface 12 can be applied, for example, by electroplating a coating of nickel or chromium, for example, as an effectively reflecting layer (the coating of the mirror surface 12 is illustrated greatly enlarged in FIG. 2 for reasons of clarity). Before applying the coating, the corresponding surface or a corresponding region of the mount 17 will be finely treated mechanically, for example by lapping.

[0028] After application of the coating, the surface is polished for good reflection. As a last step, lens seat, lower reference surface 18 and centering collar 21 are finally turned. The mirror surface 12 is thereby automatically aligned relative to the lens support during this cycle. 

1. An objective having optical elements such as lenses and mirrors, characterized in that at least some of the optical elements (16, 11 a) are provided in each case with a reflecting surface outside the optically active region, as a reference surface (12) for adjusting the optical element inside the objective or inside a subassembly of an objective.
 2. An objective having optical elements such as lenses and mirrors that are supported iii mounts, characterized in that at least some of the mounts (17) of the optical elements (16, 11 a) are provided in each case with a reflecting surface (12) for adjusting the optical element inside the objective or inside a subassembly of an objective.
 3. The objective as claimed in claim 1 or 2, characterized in that the reference surface (12) is perpendicular to the optical axis (1 a).
 4. The objective as claimed in claim 1 or 2, characterized in that the reference surface (12) is designed as a mirror surface.
 5. The objective as claimed in claim 1 or 2, characterized in that, given a lens (16) as optical element, the reference surface (12) is aligned parallel to one or more reference collars (18) of the mount (17).
 6. The objective as claimed in claim 5, characterized in that the reference surface (12) is formed by lapping and polishing a part of the mount.
 7. The objective as claimed in claim 5, characterized in that the reference surface (12) is formed by coating at least a part of the mount (17).
 8. The objective as claimed in claim 7, characterized in that the coating has nickel or chromium.
 9. An optical element, particularly a lens or mirror, characterized by a reflecting surface (12) outside the optically active region, the reflecting surface (12) being located on the optical element.
 10. An optical element, particularly a lens or mirror, having a mount, characterized by a reflecting surface (12) on the mount (17).
 11. The objective as claimed in claim 1, as a projection objective for use in semiconductor lithography for the purpose of producing semiconductor elements. 